As you have noticed, bpmedialist only shows assigned media with valid images (the only exeption to this rule are Frozen tapes that does not expire like other tapes).

Mark's suggestion is best. Command line alternative is 'vmquery -a -bx' and look for tapes with no Assign Time.

Any particular reason why you need to re-label tapes? VTL not automatically reclaiming space?
If this is the case, keep an eye on Media Summary report (in Verbose mode) to keep track of future expiry dates.

Supporting Storage Foundation and VCS on Unix and Windows as well as NetBackup on Unix and WindowsHandy NBU Links